The present invention relates to nuclear power reactors, and more particularly, to a method and apparatus for sealing leaking welds on the head of a nuclear reactor vessel.
On typical pressurized water nuclear reactors, a plurality of control rod drive mechanisms (CRDM) are mounted on the vessel head. The mechanisms are located in pressure boundary housings and provide controlling motion to the neutron absorbing control rods. The components which make up the housings are assembled to each other and joined with omega sealed welds. A canopy seal omega weld is located between a reactor vessel head nozzle and a mating part (see FIG. 1). The weld has a tendency to develop cracks as a result of stress corrosion cracking. These cracks typically propagate through the weld until leakage occurs.
It is common to have reactor vessel head nozzles located at every potential control rod position. A majority of the nozzles are used for control rods, a smaller number of nozzles are used for core-exiting thermocouple instrumentation and several other nozzles (spares) are capped. canopy seal omega weld leaks have been fixed by shutting down the nuclear plant, draining the reactor coolant to a level below the reactor vessel head and applying a weld overlay to the leaking location. Such a process takes a large amount of time, gives the workers a large dose of radioactivity due to the close proximity and contact with the reactor vessel head, and is no different in structure than the existing weld. Thus, leakage could occur again in the same location.